1. Field of the Invention
The present invention relates to an improved apparatus and method for operating dimming fluorescent lamps in a deep dimming mode, and, in particular, to a push-pull inverter circuit capable of operation in a pulse width modulated (PWM) deep dimming mode.
2. Description of the Related Art
Existing LCD back lighting systems utilize a variety of circuit topologies. Two popular circuit topologies are the half bridge inverter and buck power stage plus current-fed push-pull inverter (also referred to as a Royer inverter).
To conserve energy most LCD back lighting systems including those described above are dimmable systems. For those applications which use CCFL lamps, two dinmming methods are commonly employed. A first method is PWM power regulation, and a second method is output current regulation using frequency shift or input voltage regulation. FIG. 1 illustrates a buck power stage 2 plus current feed push-pull inverter 4 topology. This circuit topology performs the dimming function by PWM output current regulation. The buck power stage is used to regulate the output current. The output current in turn regulates the output power to perform PWM dimming. The current-fed push-pull portion does not include a power regulation function. To perform dimming, the buck power stage controls the output power which controls the amplitude of the lamp current. The efficiency of the overall circuit topology of the prior art circuit of FIG. 1 is determined by the efficiencies of the constituent stages, namely, the buck power stage and the current-fed push-pull stage. While the current-fed push-pull stage can reach a high efficiency, the buck power is inherently inefficient. A further shortcoming of the circuit is that it is not suitable for operation in a pulse width modulated deep dimming mode. To make the circuit suitable for deep dimming applications, it is necessary to convert the current fed push-pull configuration to a voltage fed push pull configuration. A voltage fed push-pull configuration is more desirable than a current fed push-pull configuration. This is required because a voltage fed push-pull configuration can respond much faster to input current changes.
FIG. 2 illustrates half-bridge type inverter circuit topology of the prior art. The half-bridge type inverter topology is a more efficient circuit topology than the buck stage/push-pull type inverter topology described above. Similar to the push-pull type inverter, the half-bridge type inverter includes a transformer T. It is well known in the art that for a half-bridge inverter circuit configuration the output voltage Vout is generally half of the input voltage, Vin. So for a 12V input voltage the maximum voltage on the primary of the transformer is 6V. However, the lamp requires a voltage on the order of 690V. As such, the turns ratio of the transformer must be greater than 100xc3x97. The high turns ratio of the transformer T reduces the efficiency of the circuit. A further shortcoming of this circuit configuration is that although the steady-state current of the load RL (i.e., lamp) is 6 milliamps, the reflected current is very high due to the transformer turns ratio. The high reflected current further serves to reduce the efficiency of the circuit.
It is an object of the present invention to provide a voltage-fed series resonant push-pull inverter that is capable of efficient operation in a PWM deep dimming mode.
According to one aspect of the present invention, there is provided a voltage-fed series resonant push-pull inverter comprising: a DC voltage source, a transformer having a first and a second primary winding and at least one secondary winding adapted to be connected in series with a lamp load; a first resonant circuit including a first resonant inductor and a resonant capacitor, one side of said first resonant inductor connected in series with said first primary winding of said transformer, the other side of said first resonant inductor being connected in series a first switching transistor and also connected to one side of said resonant capacitor;
The novel circuit further comprises: a second resonant circuit including a second resonant inductor and the resonant capacitor, one side of said second resonant inductor connected in series with said second primary winding of said transformer, the other side of said second resonant inductor being connected in series with a second switching transistor and also connected to the other side of said resonant capacitor, said resonant inductor being magnetically coupled to said first resonant inductor;
The construction of the novel circuit allows it to be rapidly switched on and off to perform deep pulse with modulated (PWM) dimming.
According to another aspect of the invention, the first and second resonant inductors are magnetically coupled to each other whereby each inductor stores energy in a respective half-switching cycle whereby the stored energy is released in the next half-switching cycle thereby providing a boost function.
According to a further aspect of the invention, the voltage fed push-pull inverter has a low input impedance and a high output impedance for driving CCFL loads and the like in a PWM deep dimming mode.
According to yet another aspect of the invention, the inventive circuit has a high Q value sufficient to breakdown a lamp load (i.e., reducing the high startup resistance), and subsequent to breaking down a lamp load the Q of the circuit transitions to a low Q value without the necessity of utilizing prior art techniques for recognizing when a lamp load transitions from the breakdown state.
One feature of the inverter of the present invention is that in situations where the load is a CCFL load or the like, the driving source is current driven to stabilize the load.